Effort dyspnea is common in patients with heart disease and is generally considered to be due to left ventricular systolic or diastolic dysfunction. To investigate a possible contribution of airways obstruction, maximum oxygen consumption was measured during, and peak expiratory flow rate measured before and immediately following treadmill exercise in 25 patients with coronary artery disease, 39 patients with microvascular dysfunction (microvascular angina) and 17 normal controls, all non-smokers. Rest and exercise ejection fraction was measured in patients by radionuclide angiography. Compared to normal volunteers, both men and women with microvascular angina and coronary artery disease had reduced maximum oxygen consumption during exercise. In women, the exercise peak expiratory flow rate was also significantly less than men, demonstrating an actual fall in peak expiratory flow rate compared to that measured at rest. The maximum oxygen consumption correlated with peak expiratory flow rate during exercise for both men and women, but did not correlate with the change in ejection fraction. Thus increased airways resistance independent of left ventricular systolic function contributes to exercise limitation in many patients with heart disease, particularly women. This may result from pulmonary congestion as a consequence of diastolic dysfunction. However, the data are also compatible with reflex bronchial changes during exercise, a phenomenon not seen in normal controls.